The present invention relates to an apparatus for detecting malfunctioning of a fuel feed system of an internal combustion engine (hereinafter referred to as an xe2x80x9cenginexe2x80x9d), and more specifically to an apparatus for detecting malfunctioning of a fuel feed system based on an output of an air-fuel (A/F) ratio sensor provided in an exhaust system of the engine.
The Japanese Laid-Open Patent Application No. 8-121226 describes a scheme for detecting malfunctioning of a fuel feed system of an engine comprising an O2 sensor for detecting an A/F ratio in exhaust gas and a purge control valve placed between a fuel tank and an air intake pipe. According to the scheme, during a failure monitoring, purging is forced to stop when a malfunction determination parameter KO2AVE, a learning value of an A/F ratio feedback coefficient, decreases below a first decision value. Then, whether or not the decrease is caused by purging is determined. If it is caused by purging, subsequent failure monitoring is suspended. When the A/F ratio feedback coefficient rises beyond a second decision value after a predetermined period has elapsed, a process determines that purging cannot affect the decision to resume the failure monitoring.
According to the scheme described above, the frequency of the failure monitoring may decrease because the monitoring does not resume until the predetermined period elapses. In addition, the monitoring may be resumed even if there is influence of purging because the monitoring is resumed in accordance with the rise of the wide-variable A/F ratio feedback coefficient beyond the predetermined decision value. Therefore, there is a need for a highly stable failure diagnosing system that is free from influence of purging.
In order to solve the above-mentioned problem, according to one aspect of the invention, a monitoring apparatus is provided for monitoring a fuel feed system of an engine having an A/F ratio controller. The controller carries out feedback control of the A/F ratio based on an output of an A/F ratio detector provided in an exhaust system of the engine.
The monitoring apparatus includes means for calculating an A/F ratio feedback coefficient based on an output of the detector and means for calculating a malfunction determination parameter based on the A/F ratio feedback coefficient. The apparatus also includes a purge cut controller for cutting purge when the parameter reaches a first decision value, and a monitor controller for suspending the monitoring when the parameter reaches a second decision value in a purge cutting state. The apparatus further includes means for estimating a value of a malfunction determination parameter that would be taken if the purge cutting had not been carried out. The estimation is performed responsive to the malfunction determination parameter reaching the second decision value and uses as an initial value the value when the parameter reached the first decision value. The monitor controller causes the monitoring to be resumed responsive to the estimated value of the malfunction determination parameter reaching the second decision value.
When the malfunction determination parameter reaches the second decision value in the purge cutting state, that is, when the fuel feed system is normally functioning but the malfunction determination parameter is determined to have decreased below the first decision value due to the influence of purging, estimation of the malfunction determination parameter starts. This estimation is carried out using as a starting value the value of the malfunction determination parameter at the time the first decision value is reached. The estimated value of the malfunction determination parameter simulates the operation of keeping monitoring the malfunction determination parameter without purge cutting after the malfunction determination parameter reached the first decision value.
When the estimated value of the malfunction determination parameter reaches the second decision value, it is determined that the condition is over in which the malfunction determination parameter decreases below the first decision value due to influence of the purging, and the failure monitoring of the fuel system is resumed.
According to the present invention, whether purging causes a decrease in the malfunction determination parameter below the first decision value is determined based on the malfunction determination parameter in accordance with the real flow of time. In addition, whether failure monitoring of the fuel system is to be resumed is determined based on the estimated value of the malfunction determination parameter which is a value of the parameter if the purge cutting is not carried out. Since the actual malfunction determination parameter is a value resulting from the purge cutting, the parameter would require a considerable period to return to a value unaffected by the purge cutting after the resumption of purging. By using the estimated value of the malfunction determination parameter, the invention allows to quickly determine whether the failure monitoring of the fuel system can be resumed. Furthermore, since the resumption of the failure monitoring is decided on the malfunction determination parameter, which is more stable than the A/F ratio feedback coefficient and its learning value, the system can operate in a stable manner.
According to another aspect of the invention, the malfunctioning determination parameter is decided in accordance with an average of the A/F ratio feedback coefficient. In addition, the monitoring apparatus further includes a determining unit, which determines that the fuel system is malfunctioning if the malfunction determination parameter fails to reach the second decision value after the purge cutting.
Since the malfunction determination parameter is decided according to the average of the A/F ratio feedback coefficient, the parameter becomes more stable than the feedback coefficient and its learning value. Therefore, stability of system control based on the malfunction determination parameter is enhanced.